1. Field of the Invention
This invention relates to cellular networks, and more specifically to security mobility between different cellular networks.
2. Background Information
Mobile wireless devices such as cellular telephones are becoming increasingly more popular. Mobile cellular devices access a cellular system or network via a base station that receives the radio signals from the mobile device and transmits them to the cellular network. Generally, before a mobile terminal or device is given access to the cellular network, the mobile device must have been authorized to have access to the cellular network. Authentication is the process of determining whether a mobile device, terminal, or station is authorized to access the cellular system. Different cellular networks have adopted different algorithms to authenticate the users of mobile terminals and compute security keys used for encrypting traffic and/or other information transmitted over the interface between the mobile station and the cellular network after authentication.
A mobile station may roam away from access to its current cellular network into an area of another cellular network. The mobile station may desire or need the active traffic between the mobile station and the first cellular network be handed off to the new cellular network. In order to allow complete intersystem interworking, handover between the two different systems must be supported. For security reasons, it is may be desired to provide encryption of the mobile station's (i.e., user's) conversations and signaling messages before and after a call (or other information transmision) has been handed over to the new system. Different cellular network systems may have different ways of computing and distributing security keys that are used to provide encryption of the traffic. The differences between the computing and distributing of security keys of different cellular network systems usually resides in the procedures, parameters and algorithms of the different systems. In order to support handoff of traffic of a mobile station from one cellular system to another, mobility (i.e., interworking) between security mechanisms of the two cellular systems must exist.
Further, due to time requirements, it is not possible to perform the normal authentication at the second cellular system during handover. Authentication is usually a time consuming process, whereas handoff is usually a time sensitive process. Since authentication is not part of the handoff procedure, the appropriate keys for the target cellular system are not available at the target network before handoff. When a mobile device hands off from one system to another, ciphering has to be maintained beyond the handoff order and, therefore, the appropriate ciphering keys need to be available to the mobile device and to the network being handed off to (i.e., the base station of the cellular network).
Therefore, a need exists to provide distribution of the appropriate encryption keys when a subscriber (user) of a mobile station hands off from one cellular system to a different cellular system while avoiding a time consuming process to compute the keys.